1. Technical Field
The present invention relates to a surface light source device and, specifically, a surface light source device for use as a backlight of a liquid-crystal display or the like.
2. Background Art
In recent years, with mobile devices having a surface light source device incorporated therein being made thinner, the surface light source device has also been demanded to be made thinner. To make the surface light source device thinner, the thickness of a light guide plate is required to be made thinner. However, even if the thickness of the flat-plate-shaped light guide plate is able to be made thinner, decreasing the height of a light source formed of an LED has a limit. For this reason, when a thin light guide plate in a flat plate shape is used, the height of the light source is larger than the thickness of an end face (a light incidence surface) of the light guide plate, thereby causing the light source placed so as to face the light incidence surface of the light guide plate to project above an upper surface of the light guide plate. With this projection of the light source above the light guide plate, rays emitted from the light source do not all enter the light incidence surface of the light guide plate, and partially leak to the outside to degrade light use efficiency.
To solve these inconveniences, the use of a light guide plate has been suggested, in which a light introducing part having a thickness larger than a light guide plate main body is provided at an end of the flat-plate-shaped light guide plate main body and an inclined surface inclined from a portion with a maximum thickness of the light introducing part toward an end of the light guide plate main body is provided to the light introducing part. Examples of a surface light source device using this light guide plate include those disclosed in Patent Document 1 and Patent Document 2.
FIG. 1 is a perspective view of a surface light source device 11 disclosed in Patent Document 1. In the surface light source device 11 of FIG. 1, a light guide plate having a light introducing part with a thickness larger than that of a light guide plate main body is used. This surface light source device 11 is formed of a light source 12 and a light guide plate 13, and the light source 12 is placed so as to face a light incidence surface 16 of the light guide plate 13. The light incidence surface has a thickness larger than the height of the light source 12. The light guide plate 13 is fabricated by integrally forming a light guide plate main body 14 having an approximately uniform thickness and a wedge-shaped light introducing part 15. On a rear surface of the light guide plate main body 14, a deflection pattern or a diffusion pattern (not shown) is formed. The light introducing part 15 has an inclined surface 17 inclined from a portion with a maximum thickness of the light introducing part 15 toward an end of the light guide plate main body 14. Also, on the inclined surface 17 of the light introducing part 15, a directional conversion pattern 18 (a light leakage prevention pattern) formed of a plurality of parallel V grooves is provided.
In this surface light source device 11, light emitted from the light source 12 enters the light introducing part 15 from the light incidence surface 16. An end face (the light incidence surface 16) of the light introducing part 15 has a thickness larger than the height of the light source 12, and therefore light emitted from the light source 12 is efficiently taken into the light introducing part 15. The light entering the light introducing part 15 is reflected from an upper surface or a lower surface of the light introducing part 15 to be guided to the light guide plate main body 14, is reflected from the deflection pattern or the diffusion pattern, and is then emitted from a light exit surface of the light guide plate main body 14 to the outside. Here, part of light entering the light introducing part 15 from the light incidence surface 16 may not be reflected from the inclined surface 17 to pass through the inclined surface 17 to leak to the outside. For this reason, the directional conversion pattern 18 is provided to the inclined surface 17, thereby decreasing light leakage from the inclined surface 17. As a result, according to the above-structured surface light source device 11, it is possible to improve light use efficiency and also make the surface light source device thinner.
However, in the surface light source device 11 provided with the thick light introducing part 15 so as to be continued to the thin light guide plate main body 14, even if the directional conversion pattern 18 formed of a plurality of V grooves is provided, light leakage occurs as depicted in FIG. 2 (rays leaking to the outside among rays entering the light introducing part 15 are indicated by broken arrows). FIG. 2 is a schematic plan view of the state in which light leakage occurs from the surface light source device 11. In FIG. 2, light L1 represents a ray emitted from a center of light emission of the light source 12, light L2 represents a ray emitted from an end of the light source 12, and a straight line C represents a light source center. The light L1 emitted from a center of light emission 12a has a light intensity high compared with that of the light L2 emitted from an end of the light source 12. FIG. 3 is a schematic view of behaviors of the light L1 with a high light intensity emitted from the center of light emission 12a. Of rays emitted from the center of light emission 12a, the light L1 emitted from the center of light emission 12a upward is reflected from the directional conversion pattern 18 as with light L11, and therefore is less prone to leakage from the inclined surface 17. By contrast, in the conventional surface light source device 11, as depicted in FIG. 3, V grooves of the same shape each having a laterally symmetrical sectional shape are repeatedly arranged in the directional conversion pattern 18, and therefore the light L1 emitted from the center of light emission 12a in a slanting direction is prone to leaking to the outside as the light L2. That is, as the position where light enters the directional conversion pattern 18 is away from the light source center C, the light L1 gradually becomes entering a front surface of the directional conversion pattern 18 at a nearly vertically angle, and becomes prone to leaking from the directional conversion pattern 18. As a result, light leakage from the directional conversion pattern 18 in a region equal to a width W of the light source 12 (not a width of a package of the light source 12 but a width of a light exit window), in particular, in its side end part, tends to be increased, and light loss is increased.
Note that while a pattern corresponding to a directional conversion pattern is formed over an entire width of a light introducing part in the surface light source device described in Patent Document 2, light of the light source shows an approximately Lambert distribution and the light intensity is small in a direction of a large angle with respect to the front, and therefore light leakage from a side end of the light guide plate does not pose much problems. Thus, even if the directional conversion pattern is provided from one side end to the other side end of the light guide plate, light leakage poses a problem only in a region approximately equal in width to the light source.